Pelletized chromium addition agents for ferro alloys production and method therefor

ABSTRACT

Chromium addition agents which may be advantageously applied in production of chromium-containing alloys can be produced by roasting at 2,350*-2,750* F. a previously pelletized mixture of Cr ore, a binder, carbon and if desired a flux. The pellets produced are hard, internally porous and have a film on the surface thereof consisting of various metal oxides. Mainly carbides of Cr and Fe are found in the inner region of the pellet.

United States Patent Kusama et a].

154] PELLETIZED CHROMIUM ADDITION AGENTS FOR FERRO ALLOYS PRODUCTION AND METHOD THEREFOR I72] lnvcntors: Fumihiko Kusama; Mlnoru Yamanaka,

both of Saitama, Japan [73] Assignee: Showa Denko Kabushiki Kaisha, Tokyo,

Japan [22] Filed: June 24, 1969 [21] App]. No.: 836,156

[52] US. Cl ..75/3

[51] Int. Cl. .C22b 1/14, C22b 39/00 [58] Field of Search ..75/3-5 [56] References Cited UNITED STATES PATENTS 2,883,278 4/1959 Douglas ..75/5

[ 51 May 9, 1972 3,153,586 10/1964 Wienert et a1. ..75/3 3,163,519 12/1964 Hanson et a1. ..75/3 UX 3,188,195 6/1965 Price v.75/3 X 3,258,327 6/1966 Smoot ..75/3 3,319,949 5/1967 Hanson et a1. ..75/3 X Primary Examiner-Allen B. Curtis AnomeyClario Ceccon 57 ABSTRACT 5 Claims, No Drawings PELLETIZED CHROMIUM ADDITION AGENTS FOR FERRO ALLOYS PRODUCTION AND METHOD THEREFOR The present invention relates to a method for producing chromium addition agents, and relates particularly to chromiurn addition agents applicable to the production of chromiumcontaining ferro alloys, chromium-containing steels, and the like.

A known and conventional chromium addition agent is disclosed in U.S. Pat. No. 2,883,278 the resulting agglomerate of which can be obtained by heating the briquetted product of a powdered mixture of chromium ore carbon and a binder at a temperature from 1,830 to 2,375 F in an inert atmosphere. However, this process meets with considerable disadvantages in that the heating process must be carried out in an inert atmosphere.

Briefly stated, the inventors of the present invention have now found that, in the production of chromium addition agents, when the pellets prepared by the addition of a binder and, if desired, of a flux to the powdered mixture of chromium ore and carbon are roasted at a temperature of from about 2,350 to about 2,750 F in an oxidizing gaseous stream while being concurrently rolled, it is not necessary any longer to retain the heating furnace in an inert atmosphere, because the surface of the pellets becomes coated with a compact and uniform film which is caused by the rolling of the green pellets while heating.

In addition, the inventors of the present invention have also found that when the surface of the pellets is coated with a film of metallic oxide, or when flux is added further to the pellets in carrying out the above mentioned roasting operation, many advantages can be obtained.

It is, therefore, the object of the present invention to provide for an advantageous method to produce chromium addition agents which can be employed in the production of various ferro-alloys or of steel.

The pellets which are considered to constitute the raw materials for the roasting operation of the present invention are mainly composed of a powdered mixture of carbon, as the reducing agent, and chromium ore. The naturally obtainable chromium ore contains from about 30 to about 65 percent by weight of up, and from about to about 30 percent by weight of FeO and is used either independently or in blending combination with one or more additional naturally obtainable chromium ore.

In accordance with the present invention, the green pellets are roasted at temperatures ranging from about 2,350 to about 2,750 F in an oxidizing gaseous stream, and therefore the carbonization reaction caused by the roasting process is carried out without melting of the pellets.

Therefore, in consideration of the granular characteristics of chromium ore and carbon composing the pellets, the speed of reaction is greatly affected. However, the size of the particles of the charged materials should be adjusted in such a manner that the amount of the particles below about 100 microns constitutes 60 percent or more of the total weight and, preferably more than 75 percent.

On the other hand, always in accordance with the present invention, the green pellets are subjected also to a rolling process during roasting and, therefore, must have a physical strength so that they cannot be disintegrated by the impact of the pellets against one another andagainst-the wall of the roasting furnace.

However, the required physical strength can be attained by comminuting the particles of the respective materials composing the green pellets into the above mentioned particles size distribution.

Consequently, when the flux is uniformly mixed to the mixture of the chromium ore and the carbon for producing the pellets, the particle size of the flux must also be adjusted to be the above mentioned particlesizes, so as to obtain the above mentioned physical strength.

Binders to be used for producing the green pellets, include bentonite, cement, tar, pitch, pulp waste liquid, water glass, molasses, sodium carboxymethyl cellulose, polyvinyl alcohols (PVA), and other materials which can lend plasticity to the raw materials from which green pellets are produced.

When the flux is added to the components of the pellets oris deposited as a film directly thereon in accordance with the prevent invention, silica and one or more of the following: lime (CaO), magnesia (MgO), and alumina (A1 0 may be used as flux components.

The above given flux components have the effect of adjusting the slag-forming components since the impurities contained in the roasted pellets and accompanying the raw materials may be easily removed into the slag and withdrawn from the final product.

7 Furthermore, the flux addition to the pellets has the additional effect that, when the pellets of the mixture of chromium ore, carbon, and flux are roasted, the flux components of the green pellets are changed to 2MgO SiO (forsten'te) and MgO- Al O,(spinel). At the same time, said forsterite and spinel are not pure, but contain CaO, MgO, A1 0 and SiO, in the form of a solid solution, when the slag formation is carried out in the reduction reaction at the above temperatures.

The mixed solid solution of forsterite and spinel does not prevent the extraction of CO gas generated from the reduction reaction between the oxides of chromium and iron and carbon within the pellet in the above mentioned roasting operation, but it makes the pellet itself into a solid compact mass and serves the function of preventing the melt-adhesion of the pellets themselves or the melt-adhesion thereof onto the walls of the furnace.

Because of the flux contained in the green pellets, the reduction of the chromium iron oxides contained in the green pellets with the carbon can be accelerated during the reduction reaction.

The reason for the above is that the contact between the chromium ore particles and the carbon particles is increased, a greater dispersion of the reactive atoms is provided and, by adjusting the process of the slag formation, the destruction of chromite crystal structures in the chromium ore is accelerated.

Furthermore, by the addition of flux to the components of the pellet, the roasting starting temperature is lowered, and a uniform thickness of the film on the surface of the roasted pellets is obtained and, therefore, the resistance to the oxidizing gas during such formation of the film against penetration into the pellets can be increased, while the density and strength of roasted pellets are increased.

When the roasted pellets are used as the addition agents in the production of chromium containing ferro-alloys, the absorption of gas in the roasted pellets is minimized because the pellets are compact than the fluxless pellets of the prior art, and the entering of gaseous components into the product alloys is considerably reduced.

The flux-coated green pellet makes the outer film layer of the pellet even harder and more compact than the simple mixture of the raw materials with the flux and, therefore, the penetration of oxidizing gas into the inner core of the pellet can be prevented during roasting.

A comparison of the breakdown strength and the specific gravity by volume of the roasted pellets obtained by our process of adding flux into the green pellets as compared with a conventional method carried out under identical roasting conditions is shown herebelow:

(l) Break-down Strength (Average Diameter of pellets Without adding flux 25 to 30 mm.)

In carrying out the pelletization of the raw materials from which green pellets are produced, the powdered chromium ore is prepared in such a manner that the particles having sizes below about 100 microns constitute more than about 60 percent by weight, and preferably more than about 75 percent by weight of the ore and the number of moles of carbon is from about 1.5 to about 2.5 times, preferably from about 1.5 to about 2.0 times the stoichiometric amount of oxygen which is necessary to reduce the C50, and FeO of the chromium ore, the carbon particles being prepared in such a manner that the particles of size smaller than about 100 microns constitute more than 60 percent by weight, preferably more than 75 percent by weight, of the total weight of carbon. The comminuted ore and carbon are uniformly mixed, and the selected binder is added thereto and the obtained mixture is then pelletized into granules of from about 15 mm to about 50 mm from originally almond shape matter into spherical objects.

The reason for controlling the amount of carbon contained in the pellets to within the above mentioned range is that good contact between the chromium ore particles and the carbon particles in the green pellets is required, and at the same time the speed of carbonization reaction between the oxides of Cr and Fe and carbon must be increased. When the molar amount of carbon is less than about 1.0 time, the gram atom of oxygen, the rate of reaction is slower, and the reduction rate is abruptly lowered. On the other hand, when it is greater than about 2.5 times, the amount of carbon remaining in the roasted pellets is rendered needlessly excessive, and it is not possible to expect a further increase in reaction rate.

In order to pelletize the powdered mixture, pan type or drum type pelletizers or briquetting machines are used and the raw materials are shaped into appropriate form ranging from almond shape substantially to spherical objects.

For example, it is possible to shape the raw materials into the pellets described above having a diameter from about 15 mm to about 50 mm, or it is possible to obtain briquettes having the desired shape and size selected from a diameter of from about 15 mm to about 50 mm by using a briquetting machine.

The dry green pellets prepared as mentioned above have a compressive strength of at least about 5.0 kg/cm and so have sufficient resistance to impact in the rolling motion of the rotary roaster.

For example, the spherical product obtained by the pelletizer and having a diameter of about 30 mm has a compressive strength greater than about 20 kg/cm and the briquettes obtained by the briquetting machine have a compressive strength greater than 30 kg/cm.

When added to Cr ore-Carbon-binder mixture or coated on the the green pellets, the flux should be added or coated in such a manner that the individual amounts shown in the following equations l) and (2) are retained in the final pellets 1.0 Mg() by woigl1t)+0.72 CaO by weight) 1.0 A1 0 by weight) =ab0ut 1.3 to about 2.0

The flux is added in powdered form in such a manner that it contains more than about 60 percent by weight, preferably more than about 75 percent, of particles below about 100 microns.

The reason for restricting the SiO, in the pellet to the 20-45 percent range, based on the total weight of the oxides of Si, Mg, Al, and Ca therein, is as follows: when the amount of SiO, is less than about 20 percent, the melting point of the slag contained in the pellet is high and the silica does not become the reaction medium between the chromium and iron oxides and the carbon in the pellet and, as a result, the reaction velocity is lowered. The high melting point of the slag prevents the hardening of the pellet during roasting, because it is necessary to carry out the melting at a low temperature-such as 1,850 F-so that there is a definite increase in the destruction of the pellet and in its rate of powdering. On the other hand, when silica is present in amounts greater than about 45 percent, the melting point thereof is rendered too low by the co-melting of slag compounds, because FeO is practically absent, and a sticking or melt-adhesion of the pellets, either among themselves or onto the walls of the kiln, occurs.

The pelletization of the mixture containing the flux is carried out in the same manner as that of the powdered mixture of chromium ore and carbon.

The film of metal oxides coated on the surface of the pellets, may consists of any one or more of the following: lime, silica, alumina, calcia, magnesia, iron oxide, and chromium oxide.

As to a typical, illustrative method for coating the above mentioned oxides on the surface of the green pellets, water is sprayed over the green pellets to wet the surface thereof, and the pellets are then rolled in the fine powder of oxides consisting of above about 60 percent by weight and preferably above about 75 percent by weight of particles smaller than about microns.

The fine powder of the above mentioned oxides is prepared into a milky solution or dispersion and the green pellets are dipped into this preparation to form the film of oxides. Another method consists of spraying the milky oxidic solution directly onto the surface of the pellets.

In this case, when a small quantity of bentonite is added to the milky solution or to the powdered oxide mixture, the adherence of the film becomes even greater.

The thickness of the film of the oxides coated on the surface of the pellets, should be from about 1.0 mm to about 5.0 mm preferably from about 1.0 mm to about 3.0 mm, because when the film is thinner than the above mentioned broader range, there is a danger of imperfections in the film, while when it is thicker than the above mentioned broader range, no additional advantage can be attained.

The above prepared green pellets are then roasted at a temperature ranging from about 2,350 to about 2,750 F while rolling the same in an oxidizing gaseous stream, in accordance with the method of the present invention.

The term "oxidizing gaseous stream used herein means the oxidative burning gaseous stream generated when the combustion is carried out by using such fuels as heavy oils, natural gas or fine carbon powder.

In the above mentioned combustion of such fuels as heavy oils, natural gas or fine carbon powder, these fuels are supplied to the furnace with additional air or oxygen in 1.5 to 2.5 times the necessary stoichiometric amount to obtain perfect combustion and to quickly combat the CO gas produced from the reaction of chromium and iron oxides with carbon in the green pellets and converted CO When the green pellets are roasted in accordance with the method of the present invention, it is preferable to use a rotary kiln, bath type rotary furnaces or equivalent heating furnaces.

In particular, when a rotary kiln is used, a shaft furnace or a similar preheating furnace is provided with one end thereof to preheat the material and exploiting the waste heat of the rotary kiln. At least 25 percent of the chromium content of the ore should be reduced so that the chromium carbide content in the final product may reach the desired level, namely, at least 15 wt. percent, if an ore containing about 45-50% C is employed. To reach higher chromium carbide levels in the pellet, either richer ores of chromium or concentrates should be employed or a greater reduction of the chromium ore utilized should be achieved.

The pellets are roasted at a temperature of from about 2,350 to about 2,750 F in an oxidizing gaseous stream while rolling the same after the preheating treatment, till the chromium oxide contained in the chromium ore can be carbonized or carbided to more than about 15 percent by weight, preferably to more than about 20 percent by weight, as shown illustratively in the examples described hereinafter.

When the chromium oxide contained in the pellets is carbonized to more than 15 percent by weight, the amount of iron carbide in the pellets already reaches a value above 8-10 percent by weight and therefore more than 15-20 percent by weight of chromium oxide can be carbonized. Accordingly,

the chromium addition agent produced by the process of this invention can be applied to the production of ferro-alloys.

The carbonization reactions between Cr O, and carbon and between FeO and carbon contained in the green pellets are started at a temperature in the neighborhood of 2,100 F, and therefore the roasting temperature can be retained to be from about 2,350 to about 2,750 F, preferably from about 2,460 to about 2,640 F. In addition, the sintering reaction on the surface of the pellets is promoted.

The conglomerating of the individual pellets or the adhesion of pellets onto the walls of the heating furnace is to be avoided.

When the pellets composed of chromium ore and carbon, prepared in accordance with one embodiment of this invention, or the pellets of chromium ore and carbon mixed with the flux or the oxide film coated pellets are calcined while rolling, the compact oxidic film is firmly coated on the surface thereof. The thus obtained film is prevented from reoxidation in an oxidizing atmosphere at high temperature since the carbides of Cr and Fe have good resistance against high temperature oxidation, and therefore it is not necessary to maintain the roasting zone in an inert atmosphere as taught in U.S. Pat. No. 2,883,278. The effects on the oxidic coating of the pellet produced by our process are remarkably superior to those obtained by the above-mentioned patented process.

When the film of metal oxides formed on the surface of green pellets is applied in accordance with this invention, the surface of the pellets is coated with a stronger and more compact film during the roasting and rolling operation, and therefore the pulverization attributable to the attrition of the pellets caused by the rolling can be further reduced. The roasted pellets composed of chromium ore carbon and, if desired, flux have the film layer composed of CaO, SiO A1 0 MgO, unreacted Cr,0 and FeO, and other metal oxides contained as impurities in the carbon ash, the chromium ore and the binder along with some amounts of carbides of Cr and Fe smaller than those resulting within the inner regions of the pellet.

The film layer is harder than the material composing the internal portion of the pellets. The internal portion of the pellet is instead more porous as far as its center and sometimes con tains cavities, and is composed of CaO, SiO A1 0 MgO, carbides of Cr and Fe, and some free carbon. The chromium carbide is more than about percent by weight of the total Cr in the roasted pellet, and the iron carbide is more than about 8 percent by weight of the total iron in the roasted pellet. When the pellet is coasted prior to roasting, such as by spraying, dipping or rolling the green pellet, the oxidic metal coating on the surface of the finished product is considerably thicker.

The roasted pellets composed of chromium ore, carbon and flux have the film layer composed of CaO, SiO A1 0 MgO, unreacted Cr O and FeO, and other metal oxides as well as impurities in the ash of carbon, chromium ore and the binder, along with some amounts of carbides of Cr and Fe smaller than those resulting within the inner regions of the roasted pellet, and the uniformly thick and hard film layer harder than the material composing the internal portion of the pellets, and the internal portion of the film as far as the center of the roasted pellet is porous and sometimes has cavities. The SiO- retained in the roasted pellet is from about percent by weight to about 45 percent based on the total weight of MgO, CaO, SiO,, and N 0 and MgO, CaO, and A1 0 are contained in amounts specified by the equation.

R: 1.0 MgO by weight) +0.72 CaO by Weight) 1.0 A1 0 by Weight) =about 1.3 to about 2.0

in order that, when the coated green pellet is roasted in a kiln, the flux components are melted at the lower temperature region in said kiln to wet the surface of the pellet, and most of the flux components penetrate into the inner portion thereof to quickly harden the pellet.

The following are typical examples to further illustrate the present invention, without restricting the scope thereof. The raw materials used in the following examples have the compositions shown shown herebelow, only the most important components being identified. All parts are to be intended to be parts by weight.

Chromium ore Cokes Cr O 48.5% Fixed carbon 88.6% FeO 18.3 Volatiles 1.8 MgO 16.0 Fe,0, 1.0 A1 0; 12.3 MgO 0.3 SiO, 3.5 A1 0, 2.4 Other 1.4 SiO 4.7 CaO 0.8 l00.0

Other 0.4 100.0

Flux (silicate) Flux (lime stone) i0, 96.0% CaCO 97.5% Fe,O;, 0.9 Other 2.5 Mg() 0.3

100.0 A1 0 1.0 CaO 0.2 Other 1.6 100.0

I Binder (bentonite) Coating of oxide SiO, 73% C 6% Al,0 14 R0 1 MgO 2 CaO 3 8 Other 1 l MgO 14 A1 0,, 9 100 Si0 31 Other 1 100.0

EXAMPLE 1 as binder.

The obtained mixture was charged into a pan-type pelletizer, and almond ball pellets with diameter of from 30 to 35 mm were made by adding a small amount of water. The pellets were dehydrated at a temperature ranging from about 140 to about 212 F by using a dryer, and then they were pre-heated by waste heat of the rotary kiln at a temperature of 1,200 130 F in the shaft furnace provided at the end of the rotary kiln. Thereafter they were moved into an oil-fired rotary kiln with air and oxygen. While rotating the rotary kiln, the roasting of the pellets was carried out at a temperature of from about 2,350 F to about 2,650 F.

The time elapsed in passing the pellets from the 2,350 F zone to the 2,650 F zone was about 60 minutes, and the roasted pellets removed from the outlet and of the rotary kiln were quickly cooled.

The obtained roasted pellets had a strong surface film layer about 1 mm thick, and 76 percent of the iron and 52.4 percent of the chromium content were reduced to carbides.

EXAMPLE 2 A mixture composed of 100 parts of finely crushed powder of chromium ore with particles smaller than about 100 microns, and 32.4 parts of fine powder of cokes containing 80 percent of the particles smaller than about 100 microns were mixed along with 6 percent of tar, and the mixture was sufiiciently kneaded by means of a heating kneader. Briquettes with the smaller diameter of 30 mm, and the longer diameter of 35 mm, were prepared.

The briquettes were pre-heated to l,650 F by the waste heat from the rotary kiln by using a Repol kiln, and then they were moved into a rotary kiln heated by burning heavy oil with sufficient oxygen and roasted at a temperature of from about 2,350 to about 2,650 F for 50 minutes.

The roasted briquettes were removed from the rotary kiln, and quickly cooled. The film layer on the surface thereof was very compact and had a thickness of about 1 .5 mm.

The amount of the reduced iron (to carbide) contained in the briquettes was 81.6 percent while the chromium reduced to carbide was 61.3 percent.

EXAMPLE 3 Three percent of bentonite and 3 percent of sodium carboxy methyl cellulose were charged into a pan-type pelletizer along with the following mixture of fine powders:

Chromium ore 100 parts (of which 85% below 100 microns) Coke powder 31 parts (80% below 100 microns) Silicate 10 parts (80% below 100 microns) Limestone 20 parts (80% below 100 microns) While adding water, the above prepared mixture was pelletized from almond shape to ball shaped pellets of diameter from 20 to 35 mm.

Most of the water contained in the above prepared pellets was removed by using a dryer at a temperature from about 140 F to about 230 F. The pellets then were preheated by waste heat of the rotary kiln in a shaft kiln provided at the end of the rotary kiln at a temperature of about l,300 F, and thereafter while rotating the rotary kiln, they were heated at a temperature from about 2,350 F to about 2,550 F for 50 minutes. The treated pellets were discharged from the rotary kiln, and were cooled.

A compact calcined layer was formed on the surface of the quickly cooled roasted products, and 68 percent of the iron, and 49.2 percent of the chromium in the pellets were reduced to the carbide form.

The result of the analysis of the slag formation was as follows (weight percent);

SiO,

The structure of the slag formation was found to be SiO 35.2% by weight 1.0 Mg+0.72 CaO :151

EXAMPLE 4 A mixture of 100 parts of chromium ore containing 80 percent of the particles below 100 microns, 32 parts of coke powder containing 80 percent of the particles below 100 microns and 5 percent of bentonite was prepared. While splashing a small amount of water thereon, the mixture was shaped from almond to ball-shaped pellets with diameter of from 25 to 35 mm. The obtained green pellets were rotated cient oxygen at a temeperature from about 2,350 F to about 2,650" F for 50 minutes.

A compact calcined film layer was formed on the roasted products with a thickness of about 1.5 mm, 83.4 percent of the iron and 62.5 percent of the chromium contained in the pellets having been reduced to the carbides of Fe and Cr.

EXAMPLE 5 (Control run without rolling treatment) Five percent of bentonite was added to a mixture of finely I powdered raw materials composed of 100 parts of chromium ore, 31 parts of coke and water in an amount sufiicient for pelletizing. The mixture was pelletized into pellets with diameter of from about 30 to about 35 mm.

After drying, the pellets were piled into the middle zone of a rotary kiln, and were roasted without rolling at a temperature from 2,190 to 2,650 F for minutes.

In this example, the rotary treatment was not carried out, and therefore the upper zone of the piled pellets was heated at a temperature higher than 2,550 F, while the temperature of the lower zone was lower by about 400 F, and the pellets were insufficiently roasted. As a result the obtained pellet is in the carbonized state-each pellet has a different degree of carbonization and clearly shows a portion thereof insufficiently carbonized.

The reductions of Cr and Fe of the upper layer of the roasted pellets to carbides were:

Chromium lron Chromium lron As shown in the result of the experiments, when the pellets were roasted without rolling the reduction to carbides of Cr and Fe is remarkably lowered.

What we claim and wish to secure by United States Letters Patent is:

l. A chromium additive pellet having an inner porous region containing substantially chromium carbides, iron carbides, free carbon, unreacted oxides of chromium, iron, silicon, magnesium, aluminum, and calcium and at least small amounts of other oxidic impurities, and having an outer hard coating consisting of the oxides of silicon, magnesium, aluminum, calcium, chromium and iron along with small amounts of other metal oxides, wherein the amount of silicon oxide in said pellet is from about 20 percent to about 45 percent based on the total weight of the oxides of silicon, magnesium, alurninum and calcium in said pellet, and wherein said oxides of magnesium, aluminum and calcium in said pellet are present in the weight ratio 1.0 Mg()+0.72 CaO 10 MgO+O.72 CaO where R is between about 1.3 and about 2.0; pelletizing the said mixture thus obtained; roasting the green pellets thus obtained at a temperature of from about 2,350 F to about 2,750 F in an oxidizing gaseous stream while rolling.

3. The method according to claim 2, wherein the number of moles of carbon admixed with said chromium ore binder and flux is from about 1.5 to about 2.5 times the gram atoms of oxygen in the C130 and FeO in the pellet and wherein the particle sizes of said chromium ore and said carbon below about l microns amount to more than 60 percent of the weight of said chromium ore and carbon.

4. Method of producing internally porous and uniformly hard oxide-coated chromium additive pellets, which comadmixing chromium ore, carbon and a binder; pelletizing the mixture obtained; coating the green pellets thus obtained with silicon oxide and at least one of the oxides of aluminum, magnesium, and calcium; said silicon oxide in said pellet being present in about 20 percent to about 45 percent by weight, based on the total weight of the said oxides of silicon, magnesium, aluminum and calcium in the said pellet, and said oxides of magnesium, aluminum, and calcium being present in the weight ratio 1.0 MgO-i-O.72 CaO E0 where R is between about 1.3 and about 2.0; roasting the green pellets thus coated at a temperature of from about 2,350 F to about 2,750 F in an oxidizing gaseous stream while rolling.

5. The method according to claim 4, wherein the number of moles of carbon admixed with said chromium ore binder is from about 1.5 to about 2.5 times the gram atoms of oxygen in the 0: 0,, and FeO in the pellet and wherein the particle sizes of said chromium ore and said carbon below about microns amount to more than 60 percent of the weight of said chromium ore and carbon.

4: wnx a 

2. Method of producing internally porous and uniformly hard oxide-coated chromium additive pellets, which comprises admixing chromium ore, carbon, a binder and a flux; said flux consisting of at least one of the oxides of magnesium, aluminum and calcium, and of from about 20 percent to about 45 percent of silica, based on the total weight of said oxides in said flux; said oxides of magnesium, aluminum and calcium in said admixture being present in the weight ratio where R is between about 1.3 and about 2.0; pelletizing the said mixture thus obtained; roasting the green pellets thus obtained at a temperature of from about 2,350* F to about 2,750* F in an oxidizing gaseous stream while rolling.
 3. The method according to claim 2, wherein the number of moles of carbon admixed with said chromium ore binder and flux is from about 1.5 to about 2.5 times the gram atoms of oxygen in the Cr2O3 and FeO in the pellet and wherein the particle sizes of said chromium ore and said carbon below about 100 microns amount to more than 60 percent of the weight of said chromium ore and carbon.
 4. Method of producing internally porous and uniformly hard oxide-coated chromium additive pellets, which comadmixing chromium ore, carbon and a binder; pelletizing the mixture obtained; coating the green pellets thus obtained with silicon oxide and at least one of the oxides of aluminum, magnesium, and calcium; said silicon oxide in said pellet being present in about 20 percent to about 45 percent by weight, based on the total weight of the said oxides of silicon, magnesium, aluminum and calcium in the said pellet, and said oxides of magnesium, aluminum, and calcium being present in the weight ratio where R is between about 1.3 and about 2.0; roasting the green pellets thus coated at a temperature of from about 2,350* F to about 2,750* F in an oxidizing gaseous stream while rolling.
 5. The method according to claim 4, wherein the number of moles of carbon admixed with said chromium ore binder is from about 1.5 to about 2.5 times the gram atoms of oxygen in the Cr2O3 and FeO in the pellet and wherein the particle sizes of said chromium ore and said carbon below about 100 microns amount to more than 60 percent of the weight of said chromium ore and carbon. 